In many diagnostic evaluations of ultrasonic images, a quantitative evaluation of the tissue's kinematic properties (velocity and deformation) improves the ability to identify dysfunctions. This kind of analysis has a particular relevance in the field of echocardiographic imaging. In this field, the assessment of the effective ventricular function requires a knowledge of numerous properties about the ventricular dynamics. A technique for evaluating velocity is known as Tissue Doppler Imaging (TDI), which allows the measurement of tissue velocity over all points in the ventricular wall. The measurement of tissue velocity helps to uncover abnormalities which are not immediately observable from tissue visualization in B-mode imaging. The measured tissue velocity provides information about rigid body displacement and contraction/distension. The contraction/distension is related to the myocardial activity. Additional features such as local strain or strain rate of the tissue can be derived from the tissue velocities.
A drawback of TDI is that only a component of tissue velocity along a scanline can be measured. Therefore, when tissue moves in a direction that is not aligned with the scanline, the Doppler velocity does not reflect the effective tissue kinematics. Only the components of strain and strain rate along the scanline can be evaluated, giving a reduced view of the local deformation state. Moreover, this limits the application of TDI to the anatomical sites that can either be imaged aligned along a scanline or that have a displacement in the direction of the scanline. In echocardiography, these anatomical sites correspond essentially to the interventricular septum and to the lateral walls in apical view.
Another consequence is that several scanlines should be acquired at the same location for TDI acquisitions, which means that the spatial resolution, i.e. the number of scanlines, is reduced if a high acquisition frame rate is to be achieved.
United State Patent Application number US2005/0070798 discloses a method of estimating tissue velocity vectors and oriented strain from a single acquisition of B-mode ultrasonic imaging data. An optical flow velocity field estimation technique is applied for providing a dense motion vector field from a sequence of at least two successive B-mode image frames without the need to acquire more image data in the Doppler mode. An evaluation of strain and strain rate data in any direction, even transverse to the scanline, can be derived from the calculated dense motion vector field. A drawback of such a method is that it does not provide any solution for visualizing the evaluated tensors of strain and strain rate data, which would be convenient for the user.